1. Field of the Invention
The present invention relates to a magnetic disc, more particularly to a magnetic disc substrate suitable for making the magnetic disc which causes substantially no sticking to a magnetic head and is excellent in wear durability and head flyability, with low costs, a method of producing the substrate, a magnetic disc using the substrate and a method of making the disc.
2. Description of Related Art
For avoiding the sticking of a magnetic head to a thin film magnetic recording disc, a substrate for the thin film magnetic disc has been required to have a properly rough surface rather than a mirror surface. For this, there have been made some proposals: Japanese Patent Kokai (Laid-Open) No. 62-248133 discloses to form concentric circular grooves called a texture on the surface of Ni-P by machining; Japanese Patent Kokai (Laid-Open) No. 60-38720 discloses to use the surface irregularities of an Al.sub.2 O.sub.3 -TiC ceramic substrate; and Japanese Patent Kokai (Laid-Open) No. 61-123016 discloses to disperse fine particles in a primary plating layer.
None of the substrates of prior art have met all requirements at the points of disc performance, cost and mass-productivity and hitherto substrates which are unsatisfactory in some properties must have been used. In this respect, the following explanation will be made.
In case of a magnetic disc which comprises an Ni-P substrate and a Cr undercoat, a magnetic medium, a protective film, and a lubricating oil film which are formed on the mirror polished surface of the substrate, lubricating oil and water gather at the area of contact between the disc and the head at the time of stopping of head, resulting in sticking phenomenon of head. This causes impossibility of rotation of disc, and rupture and deformation of parts of mechanism for holding head.
Therefore, at present, processing of concentric circular grooves called texturing is conducted on the Ni-P polished surface to roughen the surface to prevent sticking. However, this texturing have such substantial problems as mentioned below. That is, the texturing comprises cutting the surface of Ni-P by fine abrasive grains and hence burrs tend to be produced due to the plastic flow of material in texturing, or projections with which a head collides tend to be produced owing to the re-adhesion of cuttings. Therefore, it is difficult to form a surface for flying the head at a spacing of, for example, about 0.2 .mu.m over the whole surface of the disc.
Furthermore, when the Ni-P substrate is subjected to the texturing to reduce the sticking to the magnetic head, there is also an essential problem that the flyability of the magnetic head is lowered. Therefore, there is a difficulty in texturing that the surface roughness of the substrate must be controlled to so a narrow range that the stickiness and flyability of the magnetic head both are moderately satisfied.
Prior art texturing is carried out in one coaxial direction and, therefore, there is a problem that the effect of reducing sticking to the magnetic head is poor for the radial movement of the magnetic head, i.e., the operation of the head in a so-called seeking mode. Furthermore, this texturing forms banks at both the sides of the channel provided by the texturing, which banks prevent the securing of a narrow spacing between the disc and the magnetic head.
In the future, in order to increase a recording density of disc, it will become necessary to reduce stepwise the distance of the spacing and at that time the texturing carried out at present will provide various problems.
Another problem of the textured Ni-P substrate is its costs. The disc substrate which is produced by the preparation of an Al substrate, grinding of the surface of Al substrate, plating of Ni-P, polishing of the surface of the Ni-P plating and texturing of the surface requires many steps and the costs are not necessarily low. One reason therefor is that the disc substrate is a composite comprising films of two different materials Al and Ni-P. On the other hand, a substrate comprising a single material such as ceramic or glass requires fewer production steps and costs can be reduced.
As explained above, the texturing of the Ni-P layer has substantial problems in head flyability and costs. The present invention has solved these problems, as mentioned hereinafter.
Next, conventional ceramic substrates will be explained. For the substrates, a composite ceramic substrate such as Al.sub.2 O.sub.3 -TiC, as referred to above is used. The surface is once subjected to grinding and then a rugged surface is formed by chemical or physical etching utilizing a difference in properties of materials.
According to this method, a height of projections and intervals between the projections made by grinding on the finished surface are determined depending on the state of dispersion of the two materials Al.sub.2 O.sub.3 and TiC and so a high-level dispersing technique is necessary.
This Al.sub.2 O.sub.3 -TiC substrate had the following two problems. The first one is that it is heavy in weight and the other that it contains voids. With reference to the weight, the specific gravity of Al.sub.2 O.sub.3 (alumina) is about 4 which is about 1.5 time that of metallic Al. This is a property which is undesirable for the mechanism of a magnetic disc which rotates at a high speed. However, there remains a countermeasure against this problem which comprises reducing a thickness of plate, taking into consideration the high strength of ceramics.
However, the presence of voids is a substantial problem determined by the kind of material and process. That is, since a ceramic substrate is formed by sintering ceramic powders, void between particles cannot completely be removed. Therefore, a high-pressure hot pressing method has been studied, but this method still cannot completely remove the voids. Processing oils and water are liable to penetrate and remain in the voids and these cause formation of defects in films provided thereon or reduction of adhesion between films.
For the above two reasons, the substrate comprising composite ceramic has problems in the actual use thereof. On the other hand, as mentioned hereafter, the present invention provides an essentially excellent substrate material which is free from these problems.
The third method referred to above as conventional method by which fine particles are dispersed in a plating layer comprises specifically incorporating fine particles in the order of several microns or of submicron such as those of Al.sub.2 O.sub.3 and SiC into an Ni-P chemical plating solution. By this method, there is formed a film comprsiing an Ni-P matrix in which hard fine particles are dispersed.
The most serious problem of this method is that the fine particles of, for example, Al.sub.2 O.sub.3 or SiC cannot be uniformly dispersed in the plating film, resulting in dependence in particle density on places. Therefore, this method is not suitable for a magnetic disc substrate which requires uniform surface roughness and surface contour over the whole surface. Further problem is that the Ni-P becomes ferromagnetic by heating to about 250.degree. C.
On the other hand, it is known to use a strengthened glass as substrate for the magnetic disc. This glass substrate difficult to break is produced by exchanging sodium ion in the surface portion of ordinary glass by potassium ion or the like which is larger in ionic radius than sodium ion to provide on the surface a strengthened layer having compressive stress.
When the magnetic disc is produced actually using this strengthened glass substrate, the surface of the substrate must be properly roughened by similar texturing to that applied to an Ni-P plating film. However, if the surface of glass having a strong compressive stress is subjected to texturing, chipping is liable to occur and thus it is considerably difficult to form a textured surface which is uniform and is free from defects.